Sensing device

ABSTRACT

A sensing device includes a semiconductor structure, a substrate, a first electrode and a second electrode, and a heater. A sensing area arranged on the top side of the semiconductor structure. The substrate is located under the bottom side of the semiconductor. The first electrode and the second electrode are arranged on the top side of the semiconductor structure. The heater is disposed on the semiconductor structure and separated from the sensing area by a distance less than 100 μm.

RELATED APPLICATION

This application claims the benefit of US Provisional Application Ser.No. 62/497,108, filed on Nov. 7, 2016, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a sensing device and methods of makingthe same, and in particular to a sensing device having an embeddedheater.

DESCRIPTION OF THE RELATED ART

The development of a sensing device capable of obtaining preciseinformation within a short reacting time is required for the booming IoT(Internet of Things) market. Particularly, efforts to achieve highprecision (low detection limit), low current consumption (good powerefficiency), low cost of a sensing device has been continued toimplement comfortableness of living spaces, cope with a bad industrialenvironment, and manage food manufacturing processes, etc.

A method widely used in sensing device for improving the detection limitand shorten the reacting time is bonded an additional heater to heat upthe sensing device. Hence, the sensing device reacts rapidly andaccurately measures the concentration of the substances while operatingin high temperature. In addition, the substances absorbed on the sensingdevices are removed by heating at the high temperature to recover thesensing device. Therefore, the temperature characteristic of the sensingdevice directly affects the detection limit, the reacting time, therecovery time, and the like of the sensing device. However, the heaterproviding the effective heating causes a high current consumption and isnot suitable in some application.

It is an object of the current disclosure to provide a sensing devicewith an embedded heater to improve the detection limit of the sensingdevice, and shorten the reacting and recovery time under low currentconsumption operation.

SUMMARY OF THE DISCLOSURE

The following description illustrates embodiments and together withdrawings to provide a further understanding of the disclosure describedabove.

A sensing device includes a semiconductor structure, a substrate, afirst electrode and a second electrode, and a heater. A sensing areaarranged on the top side of the semiconductor structure. The substrateis located under the bottom side of the semiconductor. The firstelectrode and the second electrode are arranged on the top side of thesemiconductor structure. The heater is disposed on the semiconductorstructure and separated from the sensing area by a distance less than100 μm.

A sensing device includes a semiconductor structure, a substrate, anelectrode, and a heater. A sensing area arranged on the top side of thesemiconductor structure. The substrate is located under the bottom sideof the semiconductor. The electrode is disposed on the top side of thesemiconductor structure and exposes the sensing area. The heater isdisposed on the semiconductor structure. The sensing device has anoperating current less than 350 mA.

A sensing device includes a semiconductor structure, a substrate, anelectrode, and a heater. The substrate is located under the bottom sideof the semiconductor. The electrode is disposed on the top side of thesemiconductor structure and exposes the sensing area. The heater isdisposed on the semiconductor structure. The sensing device has adetection limit less than 10 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a top view of a sensing device in accordance with anembodiment of the present disclosure.

FIG. 1B shows a bottom view of a sensing device shown in FIG. 1A.

FIG. 1C shows a cross-sectional view taken along line A-A′ of a sensingdevice shown in FIG. 1A.

FIG. 1D shows a bottom view of a sensing device in accordance with anembodiment of the present disclosure.

FIG. 2A˜2E show bottom views of the heaters in accordance withembodiments of the present disclosure.

FIG. 3 shows a cross-sectional view of a sensing device in accordancewith another embodiment of the present disclosure.

FIG. 4 shows a cross-sectional view of a sensing device in accordancewith another embodiment of the present disclosure.

FIG. 5A shows a top view of a sensing device in accordance with anotherembodiment of the present disclosure.

FIG. 5B shows a cross-sectional view taken along line B-B′ of a sensingdevice shown in FIG. 5A.

FIGS. 6A˜6F show steps of manufacturing a sensing device in accordancewith an embodiment of the present disclosure.

FIG. 7A shows a top view of a sensing device in accordance with anembodiment of the present disclosure.

FIG. 7B shows a cross-sectional view taken along line C-C′ of a sensingdevice shown in FIG. 7A.

FIG. 8A shows a top view of a sensing device in accordance with anembodiment of the present disclosure.

FIG. 8B shows a cross-sectional view taken along line D-D′ of a sensingdevice shown in FIG. 8A.

FIG. 9 shows a cross-sectional view of a sensing device in accordancewith another embodiment of the present disclosure.

FIG. 10 shows a cross-sectional view of a sensing device in accordancewith another embodiment of the present disclosure.

FIG. 11 shows a top view of a sensing device in accordance with anotherembodiment of the present disclosure.

FIGS. 12A˜12D show steps of manufacturing a sensing device in accordancewith an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The drawings illustrate the embodiments of the application and, togetherwith the description, serve to illustrate the principles of theapplication. The same name or the same reference number given orappeared in different paragraphs or figures along the specificationshould has the same or equivalent meanings while it is once definedanywhere of the disclosure. The thickness or the shape of an element inthe specification can be expanded or narrowed.

FIGS. 1A˜FIG. 1C show sensing devices 100 in accordance with embodimentsof the present disclosure. FIG. 1A shows a top view of the sensingdevice 100. FIG. 1B shows a bottom view of the sensing device 100. FIG.1C shows a partial cross-sectional view taken along line A-A′ in FIG.1A. The sensing device 100 is a field effect transistor (FET) whichincludes two electrodes 31, 32 (for example, one is source electrode andanother is drain electrode), a functionalization layer 9, and a sensingarea 4 (not shown in FIG. 1A, underneath the functionalization layer 9)disposed between two electrodes 31, 32. If the sensing device 100 isexposed to an environmental medium (for example, air, exhaust gas) whichincludes the substance(s) to be detected (target substance), the targetsubstance can react with the sensing area 4. In detail, the targetsubstance can accumulate on the sensing area 4, and the amount of thenet surface charge on the sensing area 4 and the electric field betweenthe sensing area 4 and the electrodes 31, 32 are therefore changed.Hence, the sensing device 100 is operated in field effect transistor(FET) mode, and the current (drain-source current) between theelectrodes 31, 32 is varied with the concentration of the targetsubstance. The field effect transistor (FET) of sensing device 100 maybe selected from the group consisting of a metal-oxide-semiconductor FET(MOSFET), a metal-semiconductor FETs (MESFETs), and a high electronmobility transistor (HEMT).

The target substance is H₂, NH₃, CO, SO_(N), NO, NO₂, CO₂, CH₄, acetone,ethanol, formaldehyde, benzene such as toluene, etc. The targetsubstance can exist in the form of gas or liquid.

In one embodiment, the sensing device 100 is a HEMT. HEMT structures canbe used in a microwave power amplifier as well as a gas or liquidsensing device because of their high two-dimensional electron gas (2DEG)mobility and saturation velocity. As shown in FIG. 1C, the sensingdevice 100 includes a substrate 1, a hetero-junction structure 2 formedon the substrate 1, a source electrode 31, and a drain electrode 32located on the top side of hetero-junction structure 2, a sensing area 4also located on the top side of the hetero-junction structure 2, and aheater 5 located on the backside of the hetero-junction structure 2. Thehetero-junction structure 2 includes a first semiconductor layer 21 anda second semiconductor layer 22, and a 2DEG channel 23. The firstsemiconductor layer 21 and the second semiconductor layer 22 are made ofpiezoelectric materials and have different bandgaps. The 2DEG channel 23can be formed at/around the interface of the first semiconductor layer21 and the second semiconductor layer 22 because of the piezoelectricand spontaneous polarization effects induced there between. While thesensing device 100 operates to detect the target substance, the sourceand drain electrodes are connected to the 2DEG channel in an Ohmiccontact type, and the sensing area 4 connects the 2DEG channel inSchottky contact type. If sensing area 4 is affected by the targetsubstance, the target substance can change the charges on the sensingarea 4. Therefore, the amount of the net surface charge changing onsensing area 4 can alter the electron density in the 2DEG channel. Thedetecting signal caused by the sensing area 4 can be amplified throughthe drain-source current. The drain-source current is varied along thechange of the electron density in the 2DEG channel. In other words, thedrain-source current is varied along the concentration of the targetsubstance. Hence, the sensing device is very sensitive to the targetsubstance and the detecting signal can be easily quantified, recorded,and transmitted.

The hetero-junction structure 2 can be made of a material which canenable the generation of a 2DEG layer. The first semiconductor layer 21has at least one material different from the second semiconductor layer22, and a wide bandgap between the first semiconductor layer 21 and thesecond semiconductor layer 22. Any material, such as III-V groupmaterials, suitable for forming a heterojunction with 2DEG can be usedto form the first semiconductor layer 21 and the second semiconductorlayer 22. In order to change the 2DEG properties of heterojunction, thefirst semiconductor layer 21 and the second semiconductor layer 22 canbe doped (for example with silicon) or un-doped. The first semiconductorlayer 21 and the second semiconductor layer 22 can be made of galliumnitride (GaN), AlN, AlGaN, Al_(y)In_(z)Ga_((1-z))N (0<y<1, 0<z<1), oretc. In one embodiment. the first semiconductor layer 21 can be made ofGaN and the second semiconductor layer 22 can be made of aluminumgallium nitride (Al_(x)Ga_((1-x))N), x=0.05˜1. In other embodiments, thehetero-junction structure 2 can be made of material, such asAlGaN/InGaN/GaN, AlN/GaN, AlN/InGaN/GaN, AlGaAs/GaAs, AlGaAs/InGaAs,InAlAs/InGaAs, and InGaP/GaAs. The first semiconductor layer 21 has athickness of a range 150˜300 nm, for example 200 nm. The secondsemiconductor layer 22 has a thickness of a range 15˜45 nm, for example25˜35 nm.

Referring to FIG. 1A and FIG. 1C, the sensing area 4 is separated fromthe electrodes 31, 32 by an insulating layer 8. The electrode 31 or 32has a thickness of 2˜5 μm, for example, 3 μm. The electrode 31 or 32 ismade of one or more metallic materials, such as Au, Cu, Ti, Ni, Al, Pt,alloy thereof, or a combination thereof. The insulating layer 8 is madeof one or more dielectric materials, such as, SiO₂ or SiN_(x). Theinsulating layer 8 has a thickness of a range 500˜1500 Å, for example700˜1000 Å.

Optionally, the sensing device 100 includes a functionalization layer 9covering the sensing area 4. The functionalization layer 9 is used toenhance catalytic dissociation of the target substance. Specifically,the functionalization layer 9 can decompose one or more targetsubstances in an environmental medium and facilitate the targetsubstances diffusing rapidly to the sensing area 4. Hence thefunctionalization layer 9 is used to improve the detection limit and/orselectivity of the sensing device 100. The functionalization layer 9 iscomposed of one or more suitable materials for the target substance, forexample, platinum is suitable for H₂ detection. Other material of thefunctionalization layer 9 can be platinum, palladium, gold, nickel,iridium, or metal oxide like as SnO₂, etc. The functionalization layer 9has a thickness of a range 5˜40 nm, for example 10˜25 nm.

As shown in FIG. 1C, the substrate 1 has a trench 10 at the bottom sideopposite to the side where the hetero-junction structure 2 resides. Thetrench 10 can have a smallest width which is larger than the biggestwidth of the sensing area 4 in a cross-sectional view. In more specific,the smallest width is the width of the top end 11 of the trench 10. Thesensing area 4 is located right above the trench 10 and fully overlappedwith the trench 10. In a top view or a bottom view (as shown in FIG. 1Aor FIG. 1B), the trench 10 is located about the central part of thesensing device 100. The trench 10 and the sensing area 4 substantiallyhave similar shapes (example, a rectangle, a square) in a top view or abottom view.

As shown in FIG. 1C, the trench 10 penetrates the substrate 1. Hence,the depth of the trench 10 is substantially the same as the thickness ofthe substrate 1. Referring to the FIG. 1C, the substrate 1 has an innersurface 12 which surrounds the trench 10 and is inclined relative to thebottom surface of the hetero-junction structure 2. Therefore, the bottomend of the trench 10 has a width (area) which is larger than that of thetop end 11 of the trench 10. The width (area) of the trench 10 graduallyincreases from the top end 11 to the bottom end in a cross-sectionalview. In another embodiment, the inner surface 12 is perpendicular tothe bottom surface of hetero-junction structure 2. The central part I ofthe sensing device 100 has a thickness which is smaller than that of theouter part II. The hetero-junction structure 2 located above the trenchis not directly supported by the substrate 1. The substrate 1 can be anymaterials suitable for epitaxial growth, such as sapphire (Al₂O₃),silicon carbide (SiC), or silicon (Si). In one embodiment, the substrate1 can be made of silicon (Si). The substrate 1 has a thickness of arange 200˜400 μm, for example 300 μm.

As shown in FIG. 1D, in another embodiment, the trench 10 has a discshape in the bottom view, and the sensing area 4 is fully overlappedwith the trench 10 in the bottom view or the top view. The shape of thetrench 10 in the bottom view is not limited to rectangle or disc, canhave other shapes same or different from the sensing area 4, and thesensing area 4 is fully overlapped with the trench 10 in the bottom viewor the top view.

As shown in FIGS. 1A˜1C, the heater 5 is located in the trench 10 anddisposed on the backside of the hetero-junction structure 2 opposite tolocation where the electrodes and sensing area 4 reside. In more detail,the heater 5 is formed on the top end 11 of the trench 10. As shown inFIG. 1A and FIG. 1B, two ends of the heater 5 are directly connected tothe conducting pads 71, 72. The conducting pads 71 and 72 extend fromthe bottom side of the sensing device 100 to the top side of the sensingdevice 100 via two conducting through holes (not shown). The pair of theconducting pads 71, 72 can be electrically connected to an externalpower supply (not shown). The conducting pads 71, 72 are near the topside where the electrodes 31, 32 don't located, and the bottom sidewhich is opposite to the top side of the sensing device 100 in the topview. The positions of the conducting pads 71, 72 are not limited to thetop and bottom side of the sensing device 100 and can be arranged on anyposition of the sensing device 100 for compatible layout. The shape ofthe conducting pads 71, 72 includes, but is not limited to, a circle, asquare, or a rectangle.

As shown in FIG. 1B and FIG. 1C, the heater 5 has a shape like as ameandering line, and the two ends are arranged in opposite sides. Aplurality of the meandering portions 51 of the heater 5 is near the leftside and the right side which are the sides the electrodes 31, 32located thereon in the top view. The plurality of the meanderingportions 51 has a right angle at the corner. In an embodiment (notshown), the plurality of the meandering portions 51 of the heater 5 isnear the top side and the bottom side. The number of the meanderingportions 51 can be varied according to the width, the material, thethickness, or other electrical parameters of the heater 5. In anotherembodiment, a portion of the meandering portions 51 of the heater 5 isnear the left/right side and another portion of the meandering portions51 of the heater 5 is near the top/bottom side. During operation, theelectrical current passes through the heater 5 via the pair of theconducting pads 71, 72, and the heater 5 generates the heat to heat upthe hetero-junction structure 2 and thermal couples to the sensing area4. The more heat generated by the heater, the more operating current isneeded to drive the heater.

The heater 5 is embedded in the sensing device 100 without bondingmaterial or additional adhesive material. Moreover, the substrate 1 isopen in a space where the heater 5 located, and the heat generated bythe heater 5 can impose on the sensing area 4 without leaking to thesubstrate 1. The temperature of the sensing area 4 is close to thetemperature of the heater 5. In other words, the heater 5 does not needto operate at high operating current, the sensing area 4 can reach theestimated temperature due to less heat leaking to the substrate. Hence,the consuming power of the heater 5 can be decreased. The shortestdistance between the heater 5 and the sensing area 4 is less than 500nm, for example 350 nm, 300 nm. The sensing device overall has a loweroperating current due to the heater 5 with lower operating current. Theoperating current of the sensing device in accordance with the presentdisclosure is lower than 350 mA, for example lower than 200 mA, 150 mA,or 100 mA. The heater 5 is made of the material with the higher thermalconductivity, higher electrical resistivity, and lower coefficient ofthermal expansion, such as Molybdenum (Mo), Polysilicon, siliconcarbide, Ti, Ni, Platinum (Pt), Au, Al, Tungsten (W), SnO₂, alloythereof, or combinations thereof. The resistivity of the heater 5depends on the material, the width, the length, the shape, and thethickness. The resistance of the heater 5 has a range of the 40˜120 ohm,for example 50˜100 ohm, 60 ohm, or 74 ohm. The thickness, width, andlength of the heater 5 depend on the resistivity of the material of theheater. In an embodiment, the thickness of the heater has a range of the1˜10 μm, for example 2˜5 μm, or 3 μm.

A passivation layer 6 is located under and covers the heater 5. Thepassivation layer 6 can drive the heat generated from the heater 5 tomove upward to the sensing area 4 via the hetero-junction structure 2.In other words, the passivation layer 6 can prevent the heat from goingdownward to the bottom side of the sensing device. As shown in FIG. 1C,the top end 11 of the trench 10 has a first portion 13 which is notcovered by the heater 5, and a second portion 14 which is covered by theheater 5. The passivation layer 6 can fully cover the trench 10(including the first portion 13 and the second portion 14). In otherwords, the passivation layer 6 is fully overlapped with the trench 10 inthe bottom view or the top view. In one embodiment, the passivationlayer 6 has a contour substantially similar to the heater 5. Hence, thefirst portion 13 is not fully covered by the passivation layer 6. Inanother embodiment, the passivation layer 6 covers the heater 5 and aportion of the first portion 13. Hence, a portion of the first portion13 is exposed and another portion of the first portion 13 is covered bythe passivation layer 6 in the bottom view. The covering area of thepassivation layer 6 can vary as long as the heat generated from theheater 5 and moving upward to the sensing area 4. The passivation layer6 is made of the dielectric material, such as, SiO₂ or SiN_(x). In otherwords, the distance between the heater 5 and the sensing area 4 isdecreased because the heater 5 is not separated from the hetero-junctionstructure 2 by the substrate 1. Further, the passivation layer 6 candrive the heat to move toward the sensing area 4. Hence, the heat wastedon the substrate would be decreased, and the power consumption of theheater 5 can be lower. Consequently, the overall power consumption ofthe sensing device can also be lower.

The shape of the heater 5 is not limited to a meandering line and canhave other geometry for increasing the heat transferring to the sensingarea 4 and the temperature uniformity of the sensing area 4.

FIGS. 2A˜2E show other embodiments of heaters with different shapes.FIG. 2A shows a heater 5A which has a shape with a meandering line withtwo ends located on the same side. The heater 5A has a first portion 52Awith a straight line and a second portion 53A with a “zigzag” meanderingline. The second portion 53A has a plurality of the meandering portions51A. The meandering portion 51A has a right angle at corner (the bendingangle is substantially 90 degree). FIG. 2B shows another embodiment ofthe heater 5B which has a shape with a meandering line similar to theheater 5A. The heater 5B has a first portion 52B with a straight lineand a second portion 53B with a “zigzag” meandering line. The secondportion 53B has a plurality of the meandering portions 51B. Themeandering portion 51B has an arc shape at the turning point to avoidthe current accumulation at the turning point, and the reliability andcracking issue. FIG. 2C shows a heater 5C which has a shape with adouble spiral. The heater 5C includes a first spiral portion 52C and asecond spiral portion 53C. The first spiral portion 52C connects to thesecond spiral portion 53C via an interconnecting portion 54C. The firstspiral portion 52C or the second spiral portion 53C has a plurality ofthe sectional lines 51C. The intersection of adjacent two sectionallines 51C has an angle θ which faces the geometric center. The angle θis an obtuse angle to avoid the current accumulation at theintersection. In another embodiment, the intersection of adjacent twosectional lines 51C has an arc shape similar to FIG. 2B.

FIG. 2D shows a heater 5D which has a shape with circles. The heater 5Dhas an inner circle 52D and an outer circle 53D which are concentricwith each other. The inner circle 52D and the outer circle 53Dcollectively form a ring. A gap 51D is formed between theinterconnecting portions 54D which are used to connect the inner andouter circles 52D, 53D. The number of circles in not limited to thenumber exemplified herein and can include one or more than two circleswith a gap between the interconnecting portions. And the plurality ofthe circles is concentric with each other. FIG. 2E shows a heater 5Ewith another shape. The heater 5E has an outer circle 53E and a disc 52Esurrounded by the outer circle 53E and located around the geometriccenter. Two interconnecting portions 54E connect the outer circle 53Eand the round shape 52E. The heater is not limited to shapes describedabove. The heater can have a shape with a combination of theaforementioned shapes. The heater 5 is made of a material with a higherelectrical resistivity and a lower thermal conductivity, such as gold(Au), aluminum (Al), polysilicon, platinum (Pt), nickel (Ni), NiCr,molybdenum (Mo), tungsten (W), titanium (Ti), silicon carbide, graphite,or alloy thereof. The conducting pads 71, 72 are made of one or moremetallic materials. The metallic material includes but not limited toAl, Cu, Au, Ag, Sn, Ti, Ni, and an alloy thereof.

FIG. 3 shows a sensing device 300 in accordance with an embodiment ofthe present disclosure. The sensing device 300 is similar to the sensingdevice 100 shown in FIG. 1C, and includes a substrate 1, ahetero-junction structure 2 formed on the substrate 1, electrodes 31, 32located on the top side of hetero-junction structure 2, a sensing area 4located on the top side of the hetero-junction structure 2. Thehetero-junction structure 2 includes a first semiconductor layer 21 anda second semiconductor layer 22, and forms a two-dimensional electrongas (2DEG) channel 23 between the first semiconductor layer 21 and asecond semiconductor layer 22. The substrate 1 has a trench 10. Thetrench 10 and the hetero-junction structure 2 are located on differentsides of the substrate 1. The heater 5 is formed on the top end 11 ofthe trench 10. The passivation layer 6 covers the heater 5. The detailswidth of the trench 10, the heater 5, and the passivation layer 6 canrefer to aforementioned descriptions related to FIG. 1C. The substrate 1can be partially removed or thinned down to form the trench 10 whichdoes not pass through the substrate 10. The substrate 1 shown in FIG. 3includes a thicker portion 15 and a thinner portion 16 surrounded by thethicker portion 15. The heater 5 is located in the trench 10 and formedon the thinner portion 16. The thinner portion 16, for example, has athickness of a range 30˜100 μm. The thicker portion 15, for example, hasa thickness of a range 200˜400 μm. In one embodiment, the thickness ofthe thinner portion 16 is of 45˜55 μm, the thickness of the thickerportion 15 is of 250˜354 μm. The area of substrate 1 right above theheater 5 has a smaller thickness, and the heat generated by the heater 5can transfer to the sensing area 4 through a shorter passage. Thedistance between the heater 5 and the sensing area 4 is less than 100μm, for example 80 μm, 70 μm, or 57 μm. Hence, the heater does not needto operate at higher operating current, and the sensing area 4 can reachthe estimated temperature due to less heat leaking to the substrate. Thepower consumption of the heater 5 can be saved. Hence, the overall ofthe sensing device has a lower power consumption due to the heater isoperated at a lower operating current level. The operating current ofthe sensing device 300 in accordance with the present disclosure islower than 350 mA, for example, lower than 200 mA, 150 mA, or 100 mA.Optionally, the sensing device 300 includes a functionalization layer 9covering the sensing area 4 to enhance the selectivity and the detectionlimit. The functionalization layer 9 is separated from the electrodes31, 32 by an insulating layer 8. The details of the elements describedherein can refer to aforementioned paragraphs directed to FIG. 1C.

FIG. 4 shows a sensing device 400 in accordance with an embodiment ofthe present disclosure. The sensing device 400 is similar to the sensingdevice 300 shown in FIG. 3, and includes a substrate 1, ahetero-junction structure 2 formed on the substrate 1, electrodes 31, 32located on an top side of hetero-junction structure 2, a sensing area 4located on the top side of the hetero-junction structure 2. Thehetero-junction structure 2 includes a first semiconductor layer 21 anda second semiconductor layer 22, and can induce a 2DEG channel 23between the first semiconductor layer 21 and a second semiconductorlayer 22. The substrate 1 has a trench 10. The trench 10 and thehetero-junction structure 2 are located on different sides of thesubstrate 1. The heater 5 is formed on the top end 11 of the trench 10.The passivation layer 6 covers the heater 5. The details of the trench10, the heater 5, and the passivation layer 6 can refer toaforementioned descriptions related to FIG. 1C. The second semiconductorlayer 22 shown in FIG. 4 has a recess 20 on the side where the sensingarea 4 resides. The second semiconductor layer 22 can be thinned down toform the recess 20 and the sensing area 4. In more detail, the secondsemiconductor layer 22 has a thicker portion 221 and a thinner portion222 surrounded by the thicker portion 221. The sensing area 4 is locatedon the thinner portion 222 and lower than the topmost surface of thesecond semiconductor layer 22. The thinner portion 222 has a thicknessof a range 5˜15 nm. The thicker portion 221 has a thickness of a range15˜45 nm. In one embodiment, the thickness of the thinner portion 222 isof 6˜10 nm, the thickness of the thicker portion 221 is of 25˜35 nm.

The distance between the sensing area 4 and the 2DEG channel 23 isdecreased due to the thin-down of the second semiconductor layer 22. Theelectric field between the sensing area 4 and the electrodes 31, 32 canbe therefore caused a change by the target substances even with a lowerconcentration; for example, the concentration is less than 10 ppm. Witha lower threshold of changing the electric field, the sensing area 4becomes to be extremely sensitive to the ambient environment (gas).Hence, the sensing device can have a lower detection limit. Optionally,the sensing device 400 can include a functionalization layer 9 coveringthe sensing area 4 to further increase the selectivity and the detectionlimit. At least a portion of the functionalization layer 9 is surroundedby the second semiconductor layer 22. In one embodiment, the topmostsurface 91 of the functionalization layer 9 is lower than the topmostsurface of the thicker portion 221 of the second semiconductor layer 22.In another embodiment, the topmost surface 91 of the functionalizationlayer 9 is higher than or substantially coplanar to the topmost surfaceof the thicker portion 221 of the second semiconductor layer 22.

FIGS. 5A˜5B show a sensing device 500 in accordance with an embodimentof the present disclosure. FIG. 5A shows the top view of the sensingdevice 500. FIG. 5B shows a partial cross-sectional view taken alongline B-B′ in FIG. 5A. The sensing device 500 is similar to the sensingdevice 300 shown in FIG. 3 and includes a substrate 1, a hetero-junctionstructure 2 formed on the substrate 1, electrodes 31, 32 located on thetop side of hetero-junction structure 2, a sensing area 4 located on thetop side of the hetero-junction structure 2. The hetero-junctionstructure 2 includes a first semiconductor layer 21 and a secondsemiconductor layer 22, and can induce a 2DEG channel 23 between thefirst semiconductor layer 21 and a second semiconductor layer 22. Thesubstrate 1 has a trench 10. The trench 10 and the hetero-junctionstructure 2 are located on different sides of the substrate 1. Theheater 5 is formed on the top end 11 of the trench 10. The passivationlayer 6 covers the heater 5. The details of the substrate 1, the heater5, and the passivation layer 6 can refer to aforementioned descriptionsrelated to FIG. 1C, FIG. 3, or FIG. 4. The electrodes 31, 32 areinterdigitated with each other for increasing the current spreading,called the interdigitated electrodes (IDE), in the top view. As shown inFIG. 5A, the electrode 31 has a first portion 311 close to a side 5001of the sensing device 500, and a plurality of extending portions 312.The electrode 32 has a first portion 321 close to an opposite side 5002of the sensing device 500, and a plurality of extending portions 322.The plurality of extending portions 312 of the electrode 31 extends fromthe first portion 311 toward but not contacting the first portion 321 ofthe electrode 32. A terminal of each of the plurality of the extendingportions 312 is connected to the first portion 311. The plurality ofextending portions 322 of the electrode 32 extends from the firstportion 321 toward but not contacting the first portion 311 of theelectrode 31. A terminal of each of the plurality of the extendingportions 322 is connected to the first portion 321. The extendingportions 312, 322 have a shape with a straight line substantiallyperpendicular to the first portions 311, 321 respectively, but notlimited to. Referring to FIG. 5B, the extending portions 312 overlap atleast a portion of the extending portions 322 from an outmost side 5003to another outmost side 5004 opposite to the outmost side 5003 in thecross-sectional view. In one embodiment, the extending portions 312, 322have curved shapes. The width of the extending portions 312 is differentfrom, or same as that of the first portion 311. In one embodiment, thewidth of the extending portions 312 is smaller than the first portion311.

The electrode 31 is separated from the electrode 32 by a non-zerodistance. In more specific, an aisle 33 is formed between the electrodes31, 32 and has a meandering path. The sensing area 4 is distributed inthe aisle 33 and separated from the electrodes 31, 32 by the insulatinglayer 8. Optionally, the sensing device 500 includes a functionalizationlayer 9 covering on the sensing area 4 and located in the aisle 33. Asshown in FIG. 5B, the topmost surface of the electrodes 31, 32 arehigher than that of the functionalization layer 9. In anotherembodiment, the second semiconductor layer 22 has a recess and includesa thinner portion and a thicker portion similar to the sensing device400 shown in FIG. 4. Hence, the distance between the sensing area 4 andthe two-dimensional electron gas (2DEG) channel 23 can be decreased forimproving the detection limit.

FIGS. 6A˜6F show steps of manufacturing a sensing device in accordancewith an embodiment of the present disclosure. As shown in FIG. 6A, asubstrate 1 is provided. The hetero-junction structure 2 including afirst semiconductor layer 21 and a second semiconductor layer 22 isepitaxially grown on the substrate 1 by deposition method, such us MetalOrganic Chemical Deposition (MOCVD) or molecular beam exitaxy (MBE). Asshown in FIG. 6B, the electrodes 31, 32 are formed on thehetero-junction structure 2. Next, as shown in FIG. 6C, the insulatinglayer 8 is formed on the electrodes 31, 32, and the hetero-junctionstructure 2. The insulating layer 8 exposes a portion of the top surfaceof the electrodes 31, 32 to electrically connect to the driving power,and exposes a portion of the top surface of the hetero-junctionstructure 2 to define the sensing area 4. Optionally, the secondsemiconductor layer 22 of the hetero-junction structure 2 can be thinneddown by etching process to form a recess. Optionally, thefunctionalization layer 9 is then deposited on the sensing area 4. Then,as shown in FIG. 6D, the structure shown in FIG. 6C is reversed and thesubstrate 1 is removed or thinned down by etching to form the trench 10.Next, referring to FIG. 6E, the heater 5 with a pattern is formed on thetop end 11 of the trench 10. Then, referring to FIG. 6F, the passivationlayer 6 is disposed to cover the heater 5. At last, the structure isfaced up to form a sensing device.

FIGS. 7A˜7B show a sensing device 600 in accordance with an embodimentof the present disclosure. FIG. 7A shows the top view of the sensingdevice 600. FIG. 7B shows a partial cross-sectional view taken alongline C-C′ in FIG. 7A. The sensing device 600 includes a substrate 1, ahetero-junction structure 2 formed on the substrate 1, electrodes 31, 32located on the top side of hetero-junction structure 2, a sensing area 4located on the top side of the hetero-junction structure 2. Thehetero-junction structure 2 includes a first semiconductor layer 21 anda second semiconductor layer 22, and can induce a 2DEG channel 23between the first semiconductor layer 21 and a second semiconductorlayer 22. The heater 5 and the sensing area 4 are located on the topside of hetero-junction structure 2. The heater 5 and the electrodes 31,32 are separated and isolated from each other by the insulating layer 8.The heater 5 has two terminals 51, 52 connected to the conducting pads71, 72 which are located on the top side of the hetero-junctionstructure 2. The passivation layer 6 is sandwiched by the heater 5 andthe hetero-junction structure 2 to electrically isolate the heater 5from the hetero-junction structure 2. Referring to FIG. 7A, the heater5, the sensing area 4, the electrodes 31, 32, and the conducting pads71, 72 are on the same side and on the top side of the hetero-junctionstructure 2.

The electrodes 31, 32 have a shape similar to the sensing device 500shown in FIG. 5A. The electrode 31 has a first portion 311 close to aside 6001 of the sensing device 600 and a plurality of extendingportions 312. The electrode 32 has a first portion 321 close to anopposite side 6002 of the sensing device 600 and a plurality ofextending portions 322. The plurality of extending portions 312 of theelectrode 31 extends from the first portion 311 toward but notcontacting the first portion 321 of the electrode 32. A terminal of eachof the plurality of the extending portions 312 is connected to firstportion 311. The plurality of extending portions 322 of the electrode 32extends from the first portion 321 toward but not contacting the firstportion 311 of the electrode 31. A terminal of each of the plurality ofthe extending portions 322 is connected to the first portion 321. Inother words, the electrodes 31, 32 are interdigitated with each other inthe top view. One of plurality of the extending portions 312 and one ofplurality of the extending portions 322 are aligned in a line andseparated by a non-zero distance in the top view. The plurality of theextending portions 312 does not overlap with the plurality of theextending portions 322 from the left side 6003 to the right side 6004 inthe top view. The electrode 31 and the electrode 32 are mirror symmetricwith each other. The extending portion 312 has a straight lineperpendicular to the first portion 311. The extending portion 322 has astraight line perpendicular to the first portion 321. In anotherembodiment, the extending portion 312 (322) is not perpendicular to thefirst portion 311 (321).

As shown in FIG. 7A, the heater 5 and the sensing area 4 are located inthe area which is between the electrode 31 and the electrode 32. Theheater 5 has a first portion 54, a second portion 55, a third portion56, and two terminals 51, 52. The third portion 56 connects the firstportion 54 and the second portion 55. The terminal 51 is located at anend of the first portion 54 which is opposite to the third portion 56.The terminal 52 is located at an end of the first portion 55 which isopposite to the third portion 56. The heater 5 is connected to theconducting pads 71, 72 via the terminals 51, 52. The electrode 31 has aninner side 313 facing the electrode 32, and the electrode 32 has aninner side 323 facing the electrode 31. The first portion 54 of theheater 5 and the inner side 313 have similar profiles, and the secondportion 55 of the heater 5 and the inner side 323 have similar profiles.In more specific, the heater 5 forms in a shape similar to a loop with asmall opening 53 sandwiched by two terminals 51, 52. The heater 5 has ashape formed along the inner sides 313, 323 of the electrodes 31, 32.The area within the loop formed by the heater 5 is the sensing area 4.The sensing area 4 is surrounded by the heater 5. From the top view asshown in FIG. 7A, the heater 5 is adjacent to the sensing area 4 by adistance close to zero.

Optionally, the functionalization layer 9 is formed on the sensing area4 to improve the detection limit and the selectivity, such as thesensing device 700 shown in FIGS. 8A˜8B. FIGS. 8A˜8B show a sensingdevice 700 in accordance with an embodiment of the present disclosure.FIG. 8A shows the top view of the sensing device 700. FIG. 8B shows apartial cross-sectional view taken along line D-D′ in FIG. 8A. Thesensing device 700 is similar to the sensing device 600 shown in FIGS.7A˜7B. The sensing device 700 includes a substrate 1, a hetero-junctionstructure 2 formed on the substrate 1, electrodes 31, 32 located on thetop side of hetero-junction structure 2, a sensing area 4 located on thetop side of the hetero-junction structure 2. The heater 5 and thesensing area 4 are located on the top side of hetero-junction structure2. The heater 5 connects to the conducting pads 71, 72. The heater 5 andthe conducting pads 71, 72 are located on the top side of thehetero-junction structure 2. The passivation layer 6 is sandwiched bythe heater 5 and the hetero-junction structure 2 to electrically isolatethe heater 5 from the hetero-junction structure 2. The functionalizationlayer 9 is formed on the sensing area 4. The functionalization layer 9and the heater 5 (or the passivation layer 6) is separated by theinsulating layer 8. The distance between the heater 5 and the sensingarea 4 is less than 80 μm in the top view, for example 70 μm, 50 μm, or20 μm. In the top view, as shown in FIG. 8A, the area surrounded by theheater 5 is slightly larger than the area of the functionalization layer9 (or the sensing area 4). As shown in FIG. 8B, the thickness of thefunctionalization layer 9 is thicker than that of the passivation layer6. In another embodiment, the thickness of the functionalization layer 9is thinner than that of the passivation 6. Optionally, there is noinsulating layer 8 formed between the functionalization layer 9 and theheater 5 (or the passivation layer 6). Hence, the distance between thefunctionalization layer 9 and the heater is close to zero in the topview.

The substrate 1 of the sensing device 600, 700 can have a trench 10 asshown in FIG. 9. FIG. 9 shows a partial cross-sectional view of thesensing device 800 in accordance with an embodiment of the presentdisclosure. The electrodes 31, 32, heater 5, and the hetero-junctionstructure 2 can refer to the descriptions directed to FIGS. 7A˜7B andFIGS. 8A˜8B. The substrate 1 of the sensing device 800 is similar tothat of the sensing device 100, 200, 300, 400, or 500. The substrate 1is thinned down to form the trench 10 which is right under the sensingarea 4. The substrate 1 has a thinner portion 16 and a thicker portion15. The thinner portion 16 is surrounded by the thicker portion 15. Inanother embodiment, the substrate 1 is partially removed under thesensing area 4 to form the trench 10. The substrate 1 which is thinneddown or partially removed can avoid the heat generated by the heater 5from leaking to the substrate 1. The heater 5 can cause the powerconsumption decreased, and the sensing area 4 to reach the estimatedtemperature easily. Hence, the operating current of the sensing devicecan be lower due to the operating current of the heater is lower, forexample less than 350 mA, 200 mA, or 100 mA.

In order to enhance the detection limit, the second semiconductor layer22 of the hetero-junction structure 2 can be thinned down to form arecess, as shown in FIG. 4. FIG. 10 shows a partial cross-sectional viewof the sensing device 900 in accordance with an embodiment of thepresent disclosure. The details of the electrodes 31, 32, the heater 5,and the substrate 1 can refer to the aforementioned descriptions relatedto FIGS. 7A˜7B, FIGS. 8A˜8B, or FIG. 9. The second semiconductor layer22 shown in FIG. 10 includes a recess where the sensing area 4 resides.The second semiconductor layer 22 has a thicker portion 221 and athinner portion 222 surrounded by the thicker portion 221. The sensingarea 4 is lower than the topmost surface of the second semiconductorlayer 22 and surrounded by the second semiconductor layer 22.

FIG. 11 shows a partial cross-sectional view of the sensing device 1000in accordance with an embodiment of the present disclosure. Thesubstrate 1, and the hetero-junction structure 2 can refer toaforementioned descriptions related to the sensing device 600, 700, 800,or 900. The electrodes 31, 32, the heater 5, and the sensing area 4 ofthe sensing device 1000 are located on the top side of thehetero-junction structure 2 and concentric with each other in the topview. Optionally, the functionalization layer 9 is formed on the sensingarea 4 to enhance the selectivity and the detection limit. Referring toFIG. 11, the electrodes 31, 32 collectively formed on a first virtualcircle C1. The heater 5 is formed on a second virtual circle C2 and thefourth virtual circle C4. The sensing area 4 is formed a third virtualcircle C3 and a central area C5. C1, C2, C3, C4, and C5 are arranged insequence from outer to inner and concentric with each other in the topview. The electrode 31 has a curved portion 311 close to a side 10001 ofthe sensing device 1000. The electrode 32 has a curved portion 312 closeto an opposite side 10002 of the sensing device 1000. The curvedportions 311, 312 are separated from each other and located on the firstvirtual circle C1. The heater 5 has a first curved portion 51, a secondcurved portion 52, a third portion 53, and the interconnecting portions54 connecting the first curved portion 51, the second curved portion 52,and the third portion 53. The first portion 51 and the second portion 52are separated from each other and located on the second virtual circleC2. The third portion 53 is located on the fourth virtual circle C4. Thesensing area 4 includes a first portion 41 and a second portion 42separated from the first portion 41. The sensing area 4 is surrounded bythe heater 5. The first portion 41 of the sensing area 4 is sandwichedby the first curved portion 51, the second curved portion 52, and thethird portion 53, and located on the third virtual circle C3. The secondportion 42 of the sensing area 4 has a circular shape located on thecentral area C5 and is surrounded by fourth virtual circle C4 where thethird portion 53 of the heater 5 resides. The electrodes 31, 32, heater5, and the sensing area 4 are arranged to decrease distances between theheater 5 and the sensing area 4 for enhancing the current spreading. Thearrangement of the electrodes 31, 32, heater 5, and the sensing area 4is not limited to circular and alternating configuration as exemplifiedherein.

FIGS. 12A˜12C show steps of manufacturing a sensing device 600 inaccordance with an embodiment of the present disclosure. As shown inFIG. 12A, a substrate 1 is provided and the hetero-junction structure 2including a first semiconductor layer 21 and a second semiconductorlayer 22 is epitaxially grown on the substrate 1 by deposition method,such us Metal Organic Chemical Deposition (MOCVD) or molecular beamexitaxy (MBE). As shown in FIG. 12B, the electrodes 31, 32 are formed onthe hetero-junction structure 2. Next, as shown in FIG. 12C, theinsulating layer 8 is formed on the electrodes 31, 32, and thehetero-junction structure 2. The passivation layer 6 and the heater 5are deposited on the hetero-junction with the specific pattern to definethe region of the sensing area 4. Optionally, the second semiconductorlayer 22 of the hetero-junction structure 2 is thinned down to form arecess before forming the passivation layer 6 and the heater 5. Then,optionally, the functionalization layer 9 is deposited on the sensingarea 4. Next, as shown in FIG. 12D, optionally, the substrate 1 isremoved or thinned down by etching to form the trench 10.

The sensor resistance (Rs) is the resistance between the two electrodes,defined as Rs=(V_(C)×R_(L))/V_(out)−R_(L). V_(C) is the voltage appliedto the two electrodes. R_(L) is the loading resistor connected in serieswith the output terminal via one of electrodes. The V_(out) is thevoltage across the load resistor R_(L). The sensitivity β of the sensingdevice is defined as a ratio of a target sensor resistance to areference sensor resistance (change ratio of sensor resistance). Forexample, the reference sensor resistance Rs (30 ppm) of the sensingdevice is set to be the resistance value measured under the targetsubstance with 30 ppm concentration. The target sensor resistance Rs(100 ppm) of the sensing device is set to be the resistance valuemeasured under the target substances with 100 ppm concentration. Then,the sensitivity of the sensing device can be presented as β=Rs (100ppm)/Rs (30 ppm). V_(out) varies in accordance to the output current ofthe sensing device; hence, the sensitivity also can be presented by thevariation of the output current. When the sensitivity β is higher, thereading circuit connected to the output of the sensing device is easierto read out without complicated amplification. The detection limit isdefined as the minimum concentration of the target substance to triggerthe sensor. The detection limit of the embodiment aforementioned is lessthan 10 ppm, for example less than 7 ppm, 5 ppm, 1 ppm, or 0.5 ppm.

It will be apparent to those having ordinary skill in the art thatvarious modifications and variations can be made to the devices inaccordance with the present disclosure without departing from the scopeor spirit of the disclosure. In view of the foregoing, it is intendedthat the present disclosure covers modifications and variations of thisdisclosure provided they fall within the scope of the following claimsand their equivalents.

What is claimed is:
 1. A sensing device, comprising: a semiconductorstructure comprising a top side, a bottom side opposite to the top side,and a sensing area arranged on the top side; a substrate arranged underthe bottom side; a first electrode and a second electrode arranged onthe top side in a configuration to expose the sensing area; and a heaterdisposed on the semiconductor structure and separated from the sensingarea by a distance of less than 100 μm.
 2. The sensing device accordingto claim 1, wherein the substrate has a trench which is overlapped withthe sensing area in a top view.
 3. The sensing device according to claim2, wherein the substrate has a thinner portion and a thicker portionsurrounding the thinner portion.
 4. The sensing device according toclaim 2, wherein the trench has a top end where the heater is located,and an open end opposite to the top end.
 5. The sensing device accordingto claim 1, further comprising a passivation layer covering the heater.6. The sensing device according to claim 1, further comprising apassivation layer sandwiched by the heater and the semiconductorstructure.
 7. The sensing device according to claim 1, furthercomprising a passivation layer formed in a loop configuration to enclosethe sensing area.
 8. The sensing device according to claim 1, whereinthe first electrode and the second electrode are interdigitated witheach other in a top view.
 9. The sensing device according to claim 1,wherein the semiconductor structure has a recess formed on the top sidewhere the sensing area is located.
 10. The sensing device according toclaim 1, wherein the heater comprises a meandering line.
 11. A sensingdevice, comprising: a semiconductor structure comprising a top side, abottom side opposite to the top side, and a sensing area arranged on thetop side; a substrate arranged under the bottom side; an electrodedisposed on the top side to expose the sensing area; and a heaterdisposed on the semiconductor structure; wherein the sensing device hasan operating current less than 350 mA.
 12. The sensing device accordingto claim 11, wherein the heater is separated from the sensing area by adistance of less than 100 μm.
 13. The sensing device according to claim11, wherein the heater is located on the top side.
 14. The sensingdevice according to claim 11, further comprising a passivation layersandwiched by the heater and the semiconductor structure.
 15. Thesensing device according to claim 11, further comprising a passivationlayer formed in a loop configuration to enclose the sensing area.
 16. Asensing device, comprising: a semiconductor structure comprising a topside, a bottom side opposite to the top side, and a sensing areaarranged on the top side; a substrate arranged under the bottom side; anelectrode disposed on the top side in a configuration to expose thesensing area; and a heater disposed on the semiconductor structure,wherein the sensing device has a detection limit less than 10 ppm. 17.The sensing device according to claim 16, wherein the heater isseparated from the sensing area by a distance of less than 100 μm. 18.The sensing device according to claim 16, wherein the heater is locatedon the top side.
 19. The sensing device according to claim 16, furthercomprising a passivation layer sandwiched by the heater and thesemiconductor structure.
 20. The sensing device according to claim 16,wherein the substrate has a trench which is overlapped with the sensingarea in a top view.